The present invention relates to merged devices comprising power MOSFETs in parallel with Schottky barrier rectifiers. More particularly, the present invention relates to the merging of trench MOSFETs and trench Schottky w rectifiers into single devices, either on a single semiconductor substrate or as components in a larger integrated circuit.
Power MOSFETs (metal oxide semiconductor field effect transistors) are well-known structures and are provided in a number of configurations, including the xe2x80x9cverticalxe2x80x9d DMOS transistor configuration illustrated in FIG. 1 and the xe2x80x9ctrenchxe2x80x9d DMOS transistor configuration illustrated in FIG. 2. Each of the configurations shown includes a highly doped substrate 100 (shown as an N+ region) on which is grown a lightly doped epitaxial layer 102 (shown as an Nxe2x88x92 region), which perform the drain function for the device. P-type body regions 104 (shown as P+/P and Pxe2x88x92 regions in FIGS. 1 and 2, respectively) are provided within the epitaxial layer 102, as are source regions 112 (shown as N+ regions). The device gates consist of conductive regions 111 and oxide regions 110. A drain contact D is connected to the back surface of the semiconductor substrate 100, a source and body contact SB is connected to the source regions 112 and body regions 104, and a gate electrode G is connected to the conductive regions 111. When a potential difference is applied across the body and the gate, charges are capacitively induced within the body region 104 adjacent to the gate oxide layer 110, resulting in the formation of an N-type channel on the surface of the body region 104 adjacent to the gate of the DMOS cell. When another potential difference is applied across the source 112 and the drain 102,100, carriers flow from the source to the drain through the channel as illustrated by the arrows in FIGS. 1 and 2, and the DMOS cell is said to be in a power-on state.
Power MOSFETs like those shown in FIGS. 1 and 2 are often used in circuits that require a Schottky diode in parallel with the MOSFET. See, e.g., U.S. Pat. Nos. 4,823,172 and 6,049,108. Such a circuit configuration is shown schematically in FIG. 3. As can be seen from this figure, the low forward voltage drop of the Schottky diode 1 prevents the body-to-drain pn-junction diode 2 that is inherent in the DMOS structure from becoming forward biased when the source-to-drain voltage becomes positive. As a result, any current that does flow in the circuit of FIG. 3 under these circumstances will flow through the Schottky diode.
By preventing the body-to-drain pn-junction diode from turning xe2x80x9conxe2x80x9d, the injection of minority carriers across the body-to-drain junction is prevented. If present, such minority carriers will delay a junction diode from turning xe2x80x9coffxe2x80x9d until all the carriers are either swept across the junction or they recombine after the voltage across the junction is reversed. The associated turn-off delay time limits the maximum frequency at which the MOSFET can operate.
On the other hand, the arrangement shown in FIG. 3 allows essentially all of the current to flow through the Schottky diode. In contrast to the inherent body-to-drain pn-junction diode 2, there is no turn-off delay associated with the Schottky diode 1, because it is not a minority carrier device.
According to an embodiment of the invention, a merged device is provided, which comprises (1) a plurality of MOSFET cells that comprise: (a) a source region of first conductivity type formed within an upper portion of a semiconductor region, (b) a body region of second conductivity type formed within a middle portion of the semiconductor region, (c) a drain region of first conductivity type formed within a lower portion of the semiconductor region, and (d) a gate region provided adjacent the source region, the body region, and the drain region and (2) a plurality of Schottky diode cells disposed within a trench network, which Schottky diode cells comprise a conductor portion in Schottky rectifying contact with the lower portion of the semiconductor region. In this embodiment, at least one MOSFET cell gate region is positioned along a sidewall of the trench network and adjacent at least one Schottky diode cell.
According to another embodiment of the invention, a merged device is provided that comprises: (1) a semiconductor substrate of first conductivity type; (2) a semiconductor epitaxial layer disposed over the substrate; (3) a trench network extending into the epitaxial region from an upper surface of the epitaxial layer and forming a plurality of mesas within the device; (4) a plurality of MOSFET cells that comprise: (a) a source region of the first conductivity type disposed within one of the mesas, (b) a body region of second conductivity type disposed within the one of the mesas, wherein the body region forms a junction with the source region, (c) a drain region of first conductivity type at least partially disposed within the one of the mesas, wherein the drain region forms a junction with the body region; and (d) a gate region situated within the trench network such that it is adjacent the source region, the body region and the drain region, wherein the gate region comprises (i) an insulating region lining at least a portion of the trench network and (ii) a conductive region within the trench network adjacent the insulating region, the conductive region being separated from the source, body and drain regions by the insulating region; and (5) a plurality of Schottky dioide cells, which Schottky dioide cells are formed over bottom portions of the trench network and comprise a conductor portion that is in Schottky barrier rectifying contact with the epitaxial layer. The merged device of this embodiment is configured such that at least some of the MOSFET cell gate regions are positioned along sidewalls of the trench network adjacent the conductor portions of at least some of the Schottky diodes.
Certain preferred embodiments include one or more of the following characteristics: (a) the semiconductor is silicon, (b) the first conductivity type is n-type conductivity and the second conductivity type is p-type conductivity, (c) the gate region comprises a doped polysilicon region adjacent a silicon dioxide region, (d) the conductor comprises one or more of titanium tungsten, platinum silicide, aluminum and aluminum alloy, (e) the body region of the device comprises a heavily doped contact region, and (f) the device comprises a p-type region that is below the Schottky diode and in contact with the perimeter of the Schottky diode.
In some embodiments, at least some of the said MOSFET cells and at least some of the Schottky diode cells are arranged in a geometric configuration that is selected from an in-line square geometry, an offset square geometry, and a hexagonal geometry.
In other embodiments, at least some of the MOSFET cells are octagonal cells. For example, at least some of the MOSFET cells and at least some of the Schottky diode cells can be arranged in a geometry that comprises alternating first and second cell rows, in which the cells of the first cell rows are greater in area than the cells of the second cell rows, and in which the cells of the first cell rows are octagonal cells. The octagonal cells can be, for example, regular octagons. The MOSFET cells can be positioned, for example, in the first cell rows, and the Schottky diode cells can be positioned, for example, within the second cell rows. Cells of the second cell rows can include, for example, octagonal cells or square cells.
According to another embodiment of the invention, a merged device is provided that comprises Schottky diode cells and MOSFET cells. In this embodiment, the Schottky diode cells are located at the bottom of a trench network, while certain gate regions of the MOSFET cells are provided on sidewalls of the trench network.
According to another embodiment of the invention, a method of forming a merged device is provided. The method comprises forming a plurality of Schottky diode cells and forming a plurality of MOSFET cells, such that: (a) the Schottky diode cells are located at the bottom of a trench network, (b) the gate regions of the MOSFET cells comprise a conductive region and an insulating region, (c) certain of the gate regions are provided on sidewalls of the trench network, and (d) the conductive regions of the gate regions are formed without the aid of a masking layer, preferably by etching a doped polysilicon layer using an anisotropic etching process.
According to another embodiment of the invention, a method for designing a merged device, which comprises a plurality of Schottky diode cells and a plurality of MOSFET cells, is provided. The method comprises: (1) removing one or more source/body mesas within a trench MOSFET device design and (2) locating one or more Schottky diode cells where the removed mesa was formerly located.
One advantage of the present invention is that a merged device is provided that contains both a DMOS transistor and a Schottky diode integrated on the same substrate.
Another advantage of the present invention is that the DMOS transistor and Schottky diode portions of the merged device can be created in an integrated manufacturing process, rather than being created sequentially.
Another advantage of the present invention is that a merged device is provided that optimizes surface area utilization by incorporating DMOS transistor function into the sidewalls of the trenches that are used to provide the Schottky diode function of the device.
Another advantage of the present invention is that the geometry of the device can be selected to vary the ratio of DMOS source perimeter to Schottky diode conducting area, optimizing device performance.
Yet another advantage of the present invention is that the ratio of DMOS source perimeter to Schottky-diode conducting area can be varied across the device, optimizing device performance at edges and as a function of temperature.
Still other embodiments and advantages of the present invention will become readily apparent to those skilled in the art upon review of the Detailed Description, Examples and Claims set forth below.